Scripture tells us that, "For charity delivers from death and keeps you from entering the darkness"; and that "Water extinguishes a blazing fire: so almsgiving atones for sin." Our Lord says: "But give for alms those things which are within; and behold, everything is clean for you.”
St. Thomas Aquinas said: "Of all prayers, the most meritorious, the most acceptable to God are prayers for the dead, because they imply all the works of charity, both corporal and spiritual."
Go here to read the full article. I highly recommend this one and remember, it is not simply a nice thing to pray for the souls in purgatory, it is your duty.